Path to importance of PD-L1 status in breast cancer

Mary Jane Friedrich

June 2019—New data support testing patients for their PD-L1 immune cell status when they are diagnosed with metastatic or unresectable locally advanced triple-negative breast cancer to determine if they might benefit from a checkpoint inhibitor.

The phase three IMpassion130 study led to accelerated approval by the Food and Drug Administration for the use of atezolizumab in combination with the chemotherapy drug nab-paclitaxel for patients with metastatic triple-negative breast cancer whose tumors are infiltrated with immune cells that express PD-L1 (Schmid P, et al. N Engl J Med. 2018;​379​[22]:​2108–2121).

“It’s PD-L1 immune cells, not tumor cells. This is in contrast to what we do clinically for non-small cell lung carcinomas,” said Ashley Cimino-Mathews, MD, associate professor of pathology and oncology, Johns Hopkins University School of Medicine, and director of the breast pathology program, Johns Hopkins Hospital.

Dr. Cimino-Mathews co-presented a CAP TODAY webinar with an author of the IMpassion130 study, Leisha A. Emens, MD, PhD, director of translational immunotherapy for the Women’s Cancer Research Center and co-leader of cancer immunology and immunotherapy, UPMC Hillman Cancer Center/Magee Womens Hospital, Pittsburgh. The webinar was made possible by a promotional sponsorship from Genentech.

“It is any immune cell within the tumor area, including macrophages, neutrophils, and lymphocytes, because that’s what was scored in the study,” Dr. Cimino-Mathews said of the trial that led to the breakthrough for triple-negative breast cancer patients.

And the immune cells have to occupy greater than or equal to one percent of the tumor area. “This is not a percentage of immune cells that are positive,” she said, adding, “It harkens back to using a percentage stromal area to calculate tumor-infiltrating lymphocytes within a breast cancer.”

The big question now, she said, is which antibody or assay should be used to test PD-L1 status in breast cancer. The only FDA-approved assay now for triple-negative breast cancer (TNBC) is the Ventana SP142 assay, which is the one used in the IMpassion130 trial. Other assays are undergoing validation studies and may gain FDA approval for use in TNBC, she said.

The cellular milieu of the breast tumor microenvironment includes not only carcinoma cells but also stromal myofibroblasts and endothelial and immune cells, Dr. Cimino-Mathews noted. Immune cells can be dispersed individually or arranged in aggregates that roughly resemble the germinal centers in lymph nodes. The latter arrangement, called a tertiary lymphoid structure, is thought to play an important role in lymphocyte trafficking to and from the tumor microenvironment.

Immune cells play varying roles in the equilibrium of cancer immune surveillance and help shape the microenvironment to be pro or anti-tumorigenic. CD8-positive T cells, Th1 helper T cells, M1 macrophages, and innate immune cells such as neutrophils, if activated, can recognize neoantigens and mount an antitumor immune response that can lead to tumor elimination.

“In contrast,” she said, “a shift toward FoxP3-positive regulatory T cells, Th2 helper T cells, and M2 macrophages can actually shift this immune milieu to its pro tumorigenic environment.”

Some breast tumors contain brisk lymphocytic infiltrate throughout the stromal space. Others have little to no lymphocyte infiltration. Tumor-infiltrating lymphocytes, or TILs, are emerging as a therapeutic target.

An international TILs working group in 2015 proposed guidelines for how to measure TILs in breast cancer (Salgado R, et al. Ann Oncol. 2015;26[2]:259–271). The recommendation is to identify the actual tumor area and only score the lymphocytes or immune cells within the invasive margin of the tumor, omitting necrotic areas, and then to score the overall stromal cellularity, Dr. Cimino-Mathews said. “You don’t want to score inflammatory cells that are far outside the actual carcinoma itself.”

The guidelines also recommend scoring only the percentage of the stromal area that’s occupied by TILs. Intratumoral TILs lying within the cancer cell mass are difficult to pick up on H&E alone and thus should be excluded. Stromal TILs are easier to detect and the recommendation is to score as a continuous variable from zero to 100 percent. “In reality,” she said, “we’ve probably been using 10 percent or five to 10 percent intervals.”

Evidence of improved survival with increasing degree of inflammation is seen most strongly in triple-negative and HER2-amplified breast carcinomas, Dr. Cimino-Mathews said. She described a study of 900 patients with TNBC that showed that an increasing degree of stromal TIL is associated with improvements in disease-free survival, distant-disease-free survival, and overall survival (Pruneri G, et al. Ann Oncol. 2016;27​[2]:​249–256).

Another study evaluated whether TIL in a tumor after treatment could predict prognosis in 300 patients with TNBC who were treated with neoadjuvant chemotherapy. Patients with a high level of TIL in residual disease after treatment—greater than 60 percent of stromal space occupied by TIL—were found to have improved metastasis-free survival and overall survival (Dieci MV, et al. Ann Oncol. 2014;25​[3]:​611–618).

“So the tumor-infiltrating lymphocytes appear to be important both before and after chemotherapy,” Dr. Cimino-Mathews said.

Proteins expressed by tumor and immune cells can stimulate or inhibit the antitumor response through a complicated interplay of protein-protein interactions at the interface of antigen-presenting cells and lymphocytes, she said. Those protein interactions that inhibit an immune response are called immune checkpoint signals. Checkpoint inhibitors can overcome a cancer cell’s evasive checkpoint maneuver and expose them to immune assault. “So what we’re trying to do with checkpoint inhibition is to apply therapies that put a brake on the brake—or that release the brake—and allow for antitumor T cell activation,” Dr. Cimino-Mathews said.

Several checkpoint inhibitors have been developed to block the PD-1/PD-L1 checkpoint pathway, including atezolizumab, which is a PD-L1 antagonist that selectively targets PD-L1 to prevent interaction with the receptor PD-1 and reverse immune cell suppression. These checkpoint inhibitors are being investigated in breast cancer along with checkpoint inhibitors that target CTLA4 (cytotoxic T-lymphocyte-associated antigen) and LAG-3 (lymphocyte activation gene-3), among others.

In breast carcinomas—and this varies among tumor types—PD-L1 can be expressed either by immune cells, including TIL, the macrophages, and neutrophils, or by the carcinoma cells themselves. The PD-1 receptor is expressed on the TIL.

Why would a tumor cell express PD-L1, which is part of an immune checkpoint inhibitor pathway? One mechanism, she said, is considered an adaptive pattern of expression and can be thought of purely as the tumor’s immune evasion strategy. “So in a tumor microenvironment that has a brisk inflammatory infiltrate, some of the cancer cells will try to stop this inflammatory, antitumor response by upregulating PD-L1 expression at the leading edge, or interface, with the inflammatory cells.”

The other pattern of expression is seen in a subset of tumors, such as a class of Hodgkin lymphoma, in which the neoplastic cells constitutively express PD-L1 on every cell. “And that’s due to amplifications in genes or gene mutations that lead to tumor cell PD-L1 expression,” whether or not inflammatory cells are present.

In breast carcinomas, PD-L1 labeling is seen on immune cells and carcinoma cells. In ductal carcinomas, this labeling occurs primarily according to an adaptive pattern of expression where PD-L1 is expressed on carcinoma cells that interface with inflammatory cells. In contrast to other tumor types, in breast carcinomas, tumor cell PD-L1 labeling is relatively rare compared with immune cell labeling.

So triple-negative and HER2-positive breast carcinomas not only have the highest degree of TIL but also are most likely to be PD-L1-positive, making them the most attractive candidates for immunotherapy.